Enzootic pneumonia in swine, also called mycoplasmal pneumonia, is caused by Mhyo. The disease is a chronic, non-fatal disease affecting pigs of all ages. Infected pigs show only mild symptoms of coughs and fever, but the disease has significant economic impact due to reduced feed efficiency and reduced weight gain. Enzootic pneumonia is transmitted from pig to pig through the nasal passages by airborne organisms expelled from the lungs of infected pigs. The primary infection by Mhyo may be followed by secondary infection by other mycoplasma species (Mycoplasma hyorhinis and Mycoplasma flocculare) as well as other bacterial pathogens.
Mhyo is a small, prokaryotic microbe capable of a free living existence, although it is often found in association with eukaryotic cells because it has absolute requirements for exogenous sterols and fatty acids. These requirements generally necessitate growth in serum-containing media. Mhyo is bounded by a cell membrane, but not a cell wall. The genome of Mhyo is approximately 1,000,000 base pairs in length.
The physical association of mycoplasmas with the host cell surface is the basis for the development and persistence of enzootic pneumonia. Mhyo infects the respiratory tract of swine, colonizing the trachea, bronchi, and bronchioles. The mycoplasma produces a ciliostatic factor which causes the cilia lining the respiratory passages to stop beating. Eventually, the cilia degenerate, leaving the pig prone to infection by secondary pathogens. Characteristic lesions of purple to gray areas of consolidation are observed in infected animals. Surveys of slaughtered animals revealed lesions in 30 to 80% of swine. Results from 37 herds in 13 states indicated that 99% of the herds had hogs with pneumonia lesions typical of enzootic pneumonia. Therefore, the need for effective preventative and treatment measures are great.
Antibiotics such as tiamulin, trimethoprim, tetracyclines and lincomycin have some benefit, but are expensive and require prolonged use. Additionally, antibiotics have not been shown to effectively eliminate spread or reinfection of Mhyo. Prevention by maintaining pathogen-free herds is sometimes possible but reintroduction of Mhyo often occurs. Due to the serious economic consequences of swine pneumonia, vaccines against Mhyo and diagnostic testing methods which will indicate the presence of an infection have been sought. Vaccines containing preparations of mycoplasmal organisms grown in serum-containing medium have been marketed, but are expensive and raise concerns regarding adverse reactions induced by serum components present in the immunizing material. Other attempts to provide vaccines have not been successful, and the disease remains widespread.
What is needed is a vaccine against mycoplasma infection in swine, and a cost-effective process of producing the same. Also needed is a diagnostic test for detecting the presence of swine mycoplasma infection in swine herds.